Steel wool.
Just shoot them.
Actually, all kidding aside-
If they were exposed to that much moisture/humidity, the powder was likely compromised. How much we talking here? From the pic I'd say Remington, and not their top quality loads. Personally, I'd dispose of them.
Really doesn't take much moisture to do that to the case. I've had some shells rust like that loose inside a gun cabinet. No rust on any guns but some shells didn't look so good. Just a quick cleaning and they were gtg.